This invention relates generally to heterojunction bipolar transistors (HBT), and more particularly the invention relates to improving the safe-operating area (SOA) of such a transistor.
Heterojunction bipolar transistors (e.g. III–V compound semiconductor) are used in amplifier circuits for telecommunications applications. A major concern lies in operating the transistors in safe-operating areas (SOA) to prevent overdrive and failure of the devices. As shown in FIG. 1, the SOA is defined by two boundaries. The first boundary, SOA Boundary I, is limited by the open-emitter base-collector junction breakdown voltage, BVcbo of the transistor. This boundary sets the operating limit of the transistor at low current densities. The second boundary, SOA Boundary II, is related to the collector breakdown when substantial injected current carriers are present in the collector. This boundary is important at medium to high current levels. If one attempts to operate a HBT beyond the SOA boundaries in the non-safe operating areas as shown in the figure, the device will catastrophically fail. The conventional way to increase the collector breakdown voltage is to increase the thickness and to decrease the doping concentration of the collector. Using the approach conventional HBT's have been produced with a BVcbo of around 70 volts by using a collector with a thickness of 3 μm and a dopant concentration of 6e 15 cm−3. However, although a larger BVcbo moves SOA Boundary I to a higher Vce, the SOA Boundary II does not necessarily move to a higher collector current, Ic. In fact, breakdown always happens at a voltage smaller than BVcbo when there is large current flowing through the transistor. This is a result of the Kirk effect.
The Kirk effect results when the collector current increases to a high enough level and the number of injected electrons compensates the space charge in the collector and changes the electric field distribution. The effect happens when the effective injected charge density exceeds the background doping concentration in the collector, and the space charge changes sign and the location of the high field region moves from the base-collector junction to the collector-subcollector junction. The breakdown then is no longer controlled by the doping density in the collector alone, but also by the collector current. As Ic increases, the effective negative space charge density increases, and this causes the electric field to increase at the collector-subcollector junction, and results in a reduction of breakdown voltage. Further, decreasing of the collector doping will only improve the low current breakdown voltage but will not improve the medium and high current breakdown voltage.